Anatomy: Animals Inside

Respiration

How do humans breathe?

Humans take in oxygen and give off carbon dioxide—in fact, humans consume more than 23,670 ounces (2,959 cups or 700 liters) of oxygen a day, exhaling it as carbon dioxide (although around 15 to 18 percent of what we breathe out is still oxygen). The way oxygen and carbon dioxide gases are exchanged is relatively simple: The gases move between the small sacs in the lungs (alveoli) and the blood by diffusion—the oxygen diffusing from the alveoli into the blood, and the carbon dioxide from the blood to the alveoli. This also involves pressures (called concentration), in which the oxygen in the alveoli is kept at a higher pressure than in the blood, and carbon dioxide pressure in the alveoli is kept at a lower level than in the blood—all done by our breathing in and out.